This invention relates generally to an imaging system, and more particularly, to a reconstruction algorithm for generating images representing a moving heart.
In at least one known imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In at least one known type of imaging system, commonly known as a computed tomography (CT) system, a group of x-ray attenuation measurements, i.e., projection data, from the detector array is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
With known CT systems, projection data is collected from a helical or axial scan to generate sequential frames of images of an area, or organ, within a patient. A frame corresponds to a two dimensional slice taken through the imaged object, e.g., the patient. Typically, an operator attempts to minimize the amount of time required to generate each image by increasing the frame rate while minimizing image degradation.
At least one known CT system collects data utilizing a large flat panel digital x-ray device, or detector, having a plurality of pixels arranged in rows and columns. Each pixel includes a photosensor, such as a photodiode, that is coupled via a switching transistor to two separate address lines, a scan line and a data line. During operation, x-ray beams passing through the object are incident on the imaging device. The radiation incident on a scintillator material and the pixel photosensors measure, by way of change in the charge across the diode, the amount of light generated by x-ray interaction with the scintillator. As a result, each pixel produces a digital electrical signal that represents the intensity of an impinging x-ray beam.
To detect coronary calcification in a patient, images of the patient's heart are generated and reviewed to identify calcium deposits. This can be accomplished by detecting the average calcium concentration in a set of CT images. However, as a result of the data collection rate and the movement of the heart and the blood, the heart images may be blurred. On the other hand, the fluoroscopy mode of the digital x-ray device is capable of generating 30 frames per second or higher image rate which is sufficient to overcome blurring due to heart motion. However, the images may be difficult to view as a result of the structures which overlap over the heart. For example, the images may include ribs, a lung, and other surrounding soft tissue. These overlapping structures cause difficulty in identifying areas of calcium deposits.
To reduce the blurring of the images, it is desirable to provide an imaging system which gathers data at sufficiently high rate so that the heart motion is minimized. It would also be desirable to provide such a system which removes the overlapping structures from the images to improve the quality of the heart images.